During the final stages of the manufacture of many types of semiconductor devices, the individual dies (or chips) are mounted on a leadframe and encapsulated within a plastic or similar type molded package. The package insulates the die from environmental contaminants such as moisture and chemicals and protects the die from physical damage. The leadframe supports each die for packaging and provides lead fingers to connect the electrical devices formed in the die to a printed circuit board or other external circuitry.
Typically, leadframes are formed in strips of copper or other metal sheet stock. Several dies may be mounted on each leadframe strip. During a conventional packaging process, each die is attached to the center portion of a leadframe, or "paddle", with an adhesive. The adhesive is typically made of an epoxy, acrylic, silicone, or polyimide material that is sandwiched between the back of the die and the mounting paddle. The lead fingers are connected to bond pads formed on the die using very fine bond wires. In lead on chip ("LOC") packaging processes, the leadframe does not have a paddle and the die is mounted directly onto the lower surfaces of the lead fingers. Then, a layer of polyimide or other suitable adhesion promoting material is applied to the face of the die, and the die and the inner lead fingers (that portion of each lead finger immediately adjacent to the die) are encapsulated in a molded plastic package. A portion of each lead finger extends outside the package for connection to an external circuit. Finally, a trim and form operation is performed to sever the individual packages from the leadframe and to bend the outer lead fingers into the proper configuration.
Presently, the polyimide protective layer is drop dispensed or "daubed" onto the face of the die prior to encapsulation. This polyimide layer helps protect the die from damage during encapsulation. The polyimide layer also increases the adhesion between the mold compound and the die. Conventional methods for drop dispensing polyimide, however, do not uniformly coat the die with the polyimide. Further, this method leaves the bond wires and inner lead fingers essentially uncoated. It would be advantageous to more uniformly coat the die with polyimide or other suitable adhesion promoting material, and to simultaneously coat the bond wires and the inner lead fingers. Increasing the polyimide coverage and the uniformity of that coverage would help reduce corner cracking and other damage to the passivation layer of the die known to be caused in part by poor coverage of the die face. In addition, coating the bond wires and lead fingers will increase the adhesion of the mold compound to those components and thereby reduce the incidence of "wiggle pin" and "heal break" type package failures typically caused by delamination of the mold compound from the leadframe. Such failures are particularly common in semiconductor devices wherein the die is large relative to the overall size of the package (i.e., a large die to package size ratio).
In view of the foregoing limitations in the packaging of semiconductor devices, there is a need in the semiconductor arts for an improved coated die/leadframe and method for coating such assemblies. Accordingly, it is one object of the invention to uniformly coat the die in a die/leadframe assembly with an adhesion promoting material prior to encapsulation. It is another object of the invention to coat the bond wires, inner lead fingers and die paddle, if applicable, prior to encapsulating those components in a plastic or similar type of molded package. It is a another object to reduce package delamination, cracking and similar types of package failures and to improve adhesion between the mold compound and the lead fingers, thereby reducing the incidence of bond wire breakage and bond failure.